We present photometric and spectroscopic observations of stars in the Large Magellanic Cloud OB associations LH 101 and LH 104, located in the HII region N158, which we have also imaged. From our observations we have constructed upper H-R diagrams for these OB associations, which we find to consist mainly of three populations, one of 2-6 Myr for the stars inside the northern bubble (LH 104), and two populations in the southern HII region (LH 101), one of <= 2 Myr and the other one aged 3-6 Myr. We have obtained for LH 101 a normal IMF, with a slope of Gamma = -1.29 +/- 0.20 whereas for LH 104 the IMF is flatter with a slope of Gamma = -1.05 +/- 0.12. These IMF slopes are consistent with that of other OB associations in the LMC. Our observations reveal in the region of LH 101 the presence of both unevolved and evolved very massive stars, whose ionizing flux is in excess of that derived from our Hβ images of the HII region. The north-west nebulosity in the region of LH 101 thus appears to be matter bound. Based on observations obtained at ESO, La Silla, Chile.